Imo it feels like these writers don't juggle different parts of a story all that well.

First film gave a lot of focus to the underlying story, but the Turtles lost out a lot.

Second film reversed it while they focus on giving the Turtles a lot more attention like they should (and I like this one better because of that), but the main plot falls very weak and seems clearly rushed half way through.

The first film had a horrendous plot but there was at least a slow setup. The second didn't really have a plot, it had sub-plots that were randomly thrown in as they go. Which proves that the writers of these films not only care less about lore (2014) but care less about telling a coherent story (OotS).

Don't be presumptuous. Movies are written at three stages: pre-production, principal photography, and post-production. It's not uncommon for the actual screenplays (at least not the credited ones) to be M.I.A. from the latter two stages, not by choice either. In other words, don't blame the writers, blame the producers, directors, and even sometimes the editors.

To be fair, there is a major difference between plot and story. Fight Club doesn't really have a plot until the last act. Up until then, it is character beats and tiny subplots. The Dark Knight is mostly character beats, subplots, and short-term story beats, all "randomly thrown in as they go." Plot is nothing more than another word for a longline. It's the crap you read at the back of a DVD box.

At what point is the novelization of it written though? While there is a little that is different, it largely follows along at the same exact pace, building it up pretty good at first then once the Turtles have to run off to Brazil everything gets disappointingly rushed; quickly hitting different points in the plot to seemingly hurry up and get to the end. (Although [spoiler alert] they never defeat anyone in the book, they meet the police on the street - the end.)

Someone had to have handed the, er, novelizer, a screenplay that looked and felt largely the same, no? Leaving it probably unaffected by last minute editing decisions? (And maybe why it excludes the 'boss fight,' so to speak?)

If something in the finished film doesn't work, point your fingers to the person/people who have the most control on the film, which in this case was probably Platinum Dunes, but could be a studio exec, as well. It is not so simple as to lay the blame on the writers. For the most part, the writers are the bottom of the barrel for the above the line crew and it's not very common for a writer to get carte blanche to do what they want and in this case, I doubt the director had final cut.

A movie with the basic premise of OOTS should have been made instead, but not the Bay movies we got.

It should have followed the original trilogy/2007 film in loose continuity. It should have started with a flashback showing the origin, then Shredder being beaten by the turtles, and sent to prison- end flashback. Then, follow OOTS's sequence of events LOOSELY in plot; Shredder is being trasferred to solitary confinement in upstate New York, and Rocksteady and Bebop and in the prison convoy with shredder. A chase scene where the turtles try to stop the Foot breaking him from the prison truck, and the three criminals get free. Baxter Stockman, the foots scientist, reveals he's made contact with a an alien being named Krang, and uses Krang's info to mutate rocksteady and bebop. Shredder strikes a deal with Krang to open a portal to earth. The turtles fight the foot and the two mutants all over the globe to stop them from assembling the pieces of the portal device, and have a final battle in New York to stop the technodrome and krang from coming through.

Casey and April already know the TMNT, and the turtles have their Henson designs, and actually use their weapons on the foot soldiers. Basically anyone who has seen any version of the TMNT before this could follow it. It wouldn't make everybody happy, but it'd make the most people happy.

You mean if it had Bebop, Pizza, Cowabunga, and Krang... everything a fan would want?

There is nothing I could post in this thread that wouldn't be summed up with that, succinctly.

Most people did not hate that movie because of the Fred Wolf-isms, they hate it because of the Bay-isims. Those elements are golden under IDW's roof. Nobody wanted the OT as interpreted by Micheal Bay. They could have done a more serious take on the OT, with decent designs and good storytelling, and most people (with a few exceptions) would have at least accepted it.

In 2014, when the Nick series was in full swing, turtlemania was running high again, people had at least goodwill towards the franchise, and people were at least willing to give a tmnt film a shot, asnd everybody who was going to see this film already knew the gist of the origin.....

... a film, NOT made by Micheal Bay or Platinum Dunes, with writers, producers, and a director who cared about the franchise, with better turtle designs, a better written script, a Casey and April who look like Casey and April, the turtles already knowing Casey and April, and a flashback that tells the origin and first battle against shredder...

...under those circumstances, a movie that followed the chain of events of Out of the shadows (shredder busted out of prison, shredder makes a deal with krang to bring the technodrome to earth, shredder has Baxter create rocksteady and bebop, the turtles and the foot race all over the world finding pieces of the portal to dimension X, and a climax on the assembling technodrome to prevent Krang's war machine from appearing in NYC) without the sex and fart jokes, megan fox, whiny Casey, and good dialouge and martial arts fight sequences.... a movie under these conditions probably would've been a hit.